Gasoline distribution systems typically rely upon pipelines to deliver gasoline from refineries to tank farms. At the tank farms gasoline is stored until it is dispensed to gasoline tanker trucks. The tanker trucks deliver the gasoline to retail gasoline stations and other dispensing outlets where the gasoline is eventually delivered to an automobile, truck, or other vehicle.
A significant physical property of gasoline is its volatility, or its ability to combust. There are two principle methods for assessing the volatility of gasoline: (1) measuring the vapor-liquid ratio, and (2) measuring the vapor pressure. The Reid method is the standard test for measuring the vapor pressure of petroleum products. Reid vapor pressure (sometimes “RVP”) is related to true vapor pressure, but is a more accurate assessment for petroleum products because it considers sample vaporization as well as the presence of water vapor and air in the measuring chamber. The Reid vapor pressure of gasoline affects the ease with which gasoline is combusted, and can have a significant impact on the ease with which an automobile engine is started, especially during colder seasons when the temperature of gasoline and its corresponding volatility decreases.
In order to keep cars performing at a consistent level year-round, gasoline marketers blend agents with gasoline that increase the Reid vapor pressure and volatility of the gasoline. Butane is commonly added as a RVP modifying agent during colder months because it is more volatile than gasoline. It is also added to reduce the cost of gasoline, because it is generally less expensive than gasoline bought at the wholesale level.
The United States Environmental Protection Agency (EPA) is concerned with gasoline vapor pressure because gasoline vapor emissions from automobiles are a major component of VOC's (volatile organic compounds) in the atmosphere. Gasoline vapor emissions are of particular concern during the warmer months when gasoline is more volatile. Accordingly, the EPA has promulgated a number of regulations that govern the volatility of gasoline, and how much butane can be blended with gasoline during May 1 through September 15. For each occasion that butane is blended with gasoline during this time period, the EPA requires that the mixture be certified to ensure that it is within the volatility guidelines.
Butane has historically been blended with gasoline at several points in the gasoline distribution chain. The first opportunity to blend butane with gasoline is at the refinery, before pipelines transport the gasoline to tank farms. Refineries often add butane at the trunk line in response to changes in Reid vapor pressure demand. This process is imprecise, however, because the blended gasoline is subsequently mixed in the pipeline with other sources of gasoline of varying Reid vapor pressure. Moreover, because gasoline pipelines serve multiple regions that have variable RVP requirements, the refinery can only modify the gasoline to the lowest maximum RVP allowed by the EPA across the various regions served by the pipeline.
Butane is also added to gasoline while it is transported in the pipeline, after consolidation of various trunk lines from refineries. Typically, butane will be added to a certain volume of gasoline with a constant volatility. However, it is difficult to perform blending in the pipeline with any measure of precision. The rates of flow within the pipelines and the Reid vapor pressure of a certain volume of gasoline within the pipeline vary considerably. An additional difficulty is that the pipeline must be physically breached in at least two locations to sample the gasoline, and to add butane to the flow of gasoline. Breaching a high volume gasoline pipeline carries with it large risks that most pipeline operations would prefer to avoid.
The third point of blending is at the tank farm. When delivery of gasoline is made to a large storage tank, the RVP of the tank is measured, and sufficient butane is added to the tank to attain a desired RVP. Because tanks farms generally service a smaller area than an entire pipeline, blending at the tank farm can be more narrowly tailored to the RVP requirements of a particular region. Blending in tanks at the tank farm is performed with existing valves in tanks to introduce the butane, and thus does not entail risks similar to butane blending during gasoline transport through a pipeline.
However, blending butane at tank farms is not without its complications. Each time that gasoline is introduced to a tank, the RVP must again be measured, and butane must be added to the tank to attain a desired RVP. Often, gasoline will be dispensed to several tanker trucks before the butane can be blended, thus losing the opportunity to blend butane in those shipments. Moreover, blending butane in tanks is labor intensive and imprecise. Because the RVP of gasoline varies within a tank, several measurements must be taken throughout the tank, and considerable stirring must occur to maximize homogeneity.
All of the foregoing methods suffer from a substantial degree of imprecision. In view of this imprecision, gasoline suppliers are unable to maximize the amount of butane blended with gasoline. In particular, during summer months, government regulations currently require recertifying of gasoline each time butane is blended. The cost of this additional certification vitiates gains that could be achieved with additional blending at the tank farm.
Several methods have been attempted to improve the precision of butane blending and the predictability of Reid vapor pressure in the final product. The Grabner unit is a substantial advance in this respect. The Grabner unit (manufactured by Grabner Instruments) is a measuring device capable of providing Reid vapor pressure and liquid-vapor ratio data for a gasoline sample typically within 6–11 minutes of introducing the sample to the unit. It has been employed at some refineries to consistently measure the volatility of gasoline, and to blend butane with the gasoline based upon an allowable RVP for the gasoline. There are also other commercially available instruments for assessing the volatility of gasoline.
Although the Grabner unit can provide more accurate assessments of gasoline volatility at the refinery, a need exists for precise measurements at the final distribution point, which is the tank farm. As explained above, the current method of taking several measurements from a large gasoline tank at the tank farm is labor intensive and must be done repeatedly because of new gasoline being introduced into the tank. A further need exists in that tank farm distributors must constantly adjust their butane blending throughout the year in response to changes in temperature and concomitant changes in EPA allowable RVP levels. Finally, a need exists for the ability to blend butane with reformulated gasoline more accurately.